This invention is concerned with joining either dissimilar metallic materials, or alloys containing hardening materials, such as carbides, nitrides and intermetallic compounds. The invention is particularly directed to reducing the coalescence and growth of voids in the lower melting point material of two metallurgically joined materials having widely differing melting points. The invention is further concerned with preventing a serious depletion of a hardening agent due to a high concentration gradient resulting from the joining process.
Dissimilar metallic materials can be joined by a number of different methods, such as pressure welding, electron beam welding, and laser beam welding. These metallurgically joined materials interdiffuse when used at elevated temperatures. The interdiffusion is uneven because the atoms of the lower melting point metal possess a higher mobility and diffuse across the junction more rapidly than those atoms of the higher melting point metal moving in the opposite direction. This net flux of low melting point metal atoms across the junction is compensated by a flux of vacancies in the opposite direction. These vacancies coalesce adjacent to the junction in the lower melting point metal. This phenomenon is commonly referred to as the Kirkendall Effect.
Long diffusion thermal aging inherent in various applications of the dissimilar metal junctions can result in the coalescence of these voids into an interconnecting structure. Thus the vacuum and structural integrity of the junction, as well as its employment as a seal, can be compromised.
A problem encountered in employing dissimilar metal junctions of materials whose selection is dictated by other considerations, such as the requirements of thermionic power systems, is to prevent or inhibit the gross growth of Kirkendall voids and reduce the concentration gradient of key hardening compounds. This leads to longer life at elevated temperatures under stress conditions which is important when system lifetimes are expected to exceed three to four years without material compromise.
In the employment of dissimilar metal joints at thermionic power conversion temperatures, the Kirkendall voids become interconnecting after long periods of diffusion time. These voids form paths that lead to junction through leakage. Thus vacuum or cesium plasma envelopes in thermionic systems become compromised. Also the plane of Kirkendall voids is known to fracture easily because of its porous structure. Inasmuch as the gross formation of Kirkendall voids forms in this plane, the cross-sectional area for thermionic current is reduced. This produces I.sup.2 R losses and localized heating.
The prior art has been directed toward inhibiting or retarding the interdiffusion of the parent materials by placing a layer of a third material between them as a barrier. It is generally accepted that the melting point of the selected barrier is important because the higher it is the lower the extent of interdiffusion. Such barrier layers may reduce but do not solve the void problem. By way of example, if tungsten is coupled to columbium, no barrier of higher melting point metal exists. Also, coupling of tungsten to columbium will still result in considerable Kirkendall void formation after brief aging of 100 to 500 hours at elevated temperatures. In some applications, such as thermionic power systems, a barrier layer of a third material cannot be used.
Another problem is encountered in the case of loss of strength or the rupture of a joint of two alloys which have incorporated a filler metal. The formation of the joint produces a severe concentration gradient of the hardening agents. The filler metal will generally have an absence of these hardening agents. Consequently, a reduction of this severe concentration gradient by methods employed for the Kirkendall effect will extend the life times of such joints under stress and at elevated temperatures.